The work aims to define the conceptual categories ("case" relations) that are associated with subjects, objects, and prepositional phrases in various kinds of sentences, in the language of young children. Two methods are employed. One method compares two-year-old's imitations of model sentences in which the constituent phrases are in the normal order with their imitations of models with inverted constituent order. Since previous work has shown that children's imitations often correct the order of wrongly-ordered models, investigation of which models they tend to reorder can be used to determine their rules governing word order. In the second method, with nursery-aged children, pictures are presented with a simple sentence describing the scene. The children are taught to put counters of different shapes on the objects in the pictures according to the role they play in the scene (e.g., one counter for the actor, another for the location, etc.). They are then given generalization trials in which the pictures and sentences are selected to uncover what properties an object must have to fall within the rule categories under study. Several studies have been conducted, or are in progress, exploring locatives, and categories underlying subject and object positions.